1. Field of the Invention
The invention relates to the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then reqenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500.degree. .C-900.degree. C., usually 600.degree. C.-750.degree. C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. Although there are myriad individual variations, typical designs are shown in U.S. Pat. No. 3,849,291 (Owen) and U.S. Pat. No. 3,894,934 (Owen et al), and U.S. Pat. No. 4,368,114 (Chester et at.) which are incorporated herein by reference.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units of this type are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many other recent patents. The H.E.R. design is used in most new units because it permits operation of an FCC with less catalyst inventory (and hence less catalyst loss and lower catalyst makeup), and because such units tend to have both less CO emissions and less NOx emissions than the single dense bed regenerators.
Unfortunately, it has not been economically justifiable to convert older style, single dense bed regenerators to the modern H.E.R. design because of the high capital cost associated with simply scrapping the old single bed regenerator. Attempts to simply use the old single stage regenerator as part of a modern two stage, H.E.R. design have not been too successful, as the old single stage units are much larger than either of the beds in an H.E.R. unit. Another complication has been that many of the older units do not have cyclone separators which are adequate to deal with the increased catalyst traffic associated with high efficiency regenerators.
The problems are especially severe in those units with a central catalyst withdrawal point. Typically the catalyst is added to impart a swirling motion to spent catalyst in the regenerator, with regenerated catalyst withdrawn from the center, or near the center of the regenerator vessel. Those units using an overflow well, to minimize bypassing of spent catalyst have severe flow distribution problems, and also devote much of the volume of the regenerator bed to holding the overflow well. This represents a significant loss of some of the most active volume available in the bubbling dense bed for regeneration of spent catalyst.
To increase the coke burning capacity of these older units, and to minimize CO emission, many now use CO combustion promoters. This reduces CO emissions, but usually increases nitrogen oxides (NOx) in the regenerator flue gas. It is difficult in a catalyst regenerator to completely burn coke and CO in the regenerator without increasing the NOx content of the regenerator flue gas. The problem of NOx emissions is more severe in bubbling dense bed regenerators than in high efficiency regenerators because there can be localized high oxygen concentrations due to passage of much of the regeneration gas through the bed in the form of relatively large bubbles. Poor spent catalyst flow patterns make the NOx problem even worse, in that excessive amounts of air are needed to achieve complete CO combustion, but this increased air flow rate makes NOx emissions worse. Heavier feeds, which generally have even higher nitrogen levels, only make the problems of adequate coke burning capacity and NOx emissions even worse.
We wanted a way to improve the efficiency of catalyst regeneration in bubbling dense bed regenerators, in a way that would permit more coke to be burned. We want to use most, if not all, of the existing regenerator vessel, and in such a way that fines in the flue gas from the unit would not be increased, so that no additional stages of cyclone separation would be required. We knew this called for contradictory steps, because increased air rates which improve fluidization or more coke burning increase particulates emissions.
We discovered a way to overcome many of the deficiencies of bubbling dense bed regenerators while retaining their admirably low particulates emissions characteristics.